Metal complex compounds are being used increasingly as catalysts in commercial processes. Important examples of reactions which are catalyzed by complex compounds are hydrogenation, hydroformylation, and polymerization. The metal complex compounds can be used alone, but frequently are combined with co-catalysts. In other cases they are employed in conjunction with excess ligands to increase their stability.
The hydroformylation process described in DE-PS 26 27 354 is an example of a reaction catalyzed by metal complex compounds. The catalyst system consists of a water-soluble rhodium complex with a water-soluble organic phosphane as a ligand and excess water-soluble phosphane. The water-solubility of the phosphane ligands is due to the presence of sulfonic acid groups in the molecule. They are preferably used in the form of alkali sulfonates, ammonium sulfonates, or alkaline earth sulfonates. The process particularly excels in its high selectivity towards the formation of straight-chain aldehydes. Furthermore, it avoids the formation of significant amounts of high-boiling by-products. For their use as catalysts, the compounds must often be available as pure metal complexes. Therefore, the manufacturing process must be followed by purification which is as efficient and loss-free as possible.
Furthermore, spent catalysts should be regenerated and reactivated in a simple manner. Thus, in the hydroformylation process described in DE-PS 26 27 354, the efficiency of the catalyst system in the formation of straight-chain aldehydes with high selectivity decreases over time with continuous operation or repeated use of the same catalyst solution.
This loss in selectivity has various causes; i.e. catalyst poisons, such as iron carbonyl, which result from the synthesis gas acting on the synthesis gas transport pipes or the reactor material, and the formation of undesired higher-boiling condensation products from the aldehydes. The selectivity is also reduced by the decrease in the ratio of phosphane to rhodium during prolonged use of the catalyst system; this is a result of degradation and oxidation processes to which the sulfonated phosphanes are subjected. These reactions lead, for example, to sulfonated phosphane oxides, phosphane sulfides, aromatic sulfonic acids, and disulfophenylphosphinic acid, in each case in the form of the respective salt. Moreover, rhodium-containing cluster compounds can also be produced. Similar conversion and degradation reactions are also observed when lipophilic catalyst systems are used; they contain arylphosphanes, e.g. triphenylphosphane, as ligands. Neither phosphane oxides, phosphane sulfides, nor the salts of aromatic sulfonic acids and disulfophenylphosphinic acid act as catalysts alone or together with rhodium. The same also applies to the cluster compounds of rhodium.
It is therefore appropriate to replace the aqueous catalyst solution which contains such cleavage products either partially or completely by fresh solution from time to time. Apart from the above-mentioned conversion and degradation products of the sulfonated phosphanes, the spent catalyst solution contains rhodium as a complex compound, as well as excess water-soluble salts of sulfonated phosphanes and impurities which are brought in with the reactants. To ensure the cost effectiveness of the process, it is desirable to recover both the rhodium complex and the excess active phosphanes.
The purification of metal complex compounds as part of the manufacturing process and the regeneration of metal complex compounds used as catalysts are the subjects of various publications. A process for recovering catalyst systems containing water-soluble rhodium compounds, sulfonated organic phosphanes, and cations is described in DE 32 35 029 Al. In this process, an amount of acid at least equivalent to the acid groups present is first added to the aqueous solution of the catalyst system. Then extraction is performed with an amine which is dissolved in an organic solvent. The organic phase, containing the amine salt of the sulfonated phosphane, is separated and treated with an aqueous solution of an inorganic base, such as NaOH.
The inactive conversion and degradation products of the sulfonated triarylphosphane can be removed by adjusting the pH to a suitable value. An aqueous solution of the phosphane sulfonate and rhodium complex is obtained which can be reused as a catalyst solution either directly, after dilution with water, or after the addition of sulfonated phosphane. This process always provides a mixture of the rhodium complex and the excess ligand. Selective separation of the rhodium complex and the other components is therefore possible only to a limited extent.
DE-OS 19 12 380 relates to a process for separating, by the use of cellulose membranes, coordination complexes of transition metals from a homogeneous free-flowing mixture of complexes with one or more organic components. This procedure is also used to separate rhodium complex compounds from mixtures which contain the reaction products from hydroformylation of low molecular weight olefins in a homogeneous phase. The rhodium complex compounds are insoluble in water and no excess ligands are present. Separation is limited to splitting the mixture into the rhodium compound and the other components, in particular olefins and aldehydes.
Aqueous solutions, containing rhodium complex compounds and water-soluble ligands in addition to other components, can also be recovered using membranes by reverse osmosis, ultrafiltration, or microfiltration. In this manner, about 50% of the salts contained in the solution can be separated, while 99% of the complex-bound rhodium is retained. Apart from the required extreme dilution of the feed solution, another disadvantage of this procedure is that some of the unchanged ligands are also lost with the precipitated salts and must be recovered in an additional process step.
Although sulfonated and thus water-soluble phosphanes were first reported as complex ligands 30 years ago by Chatt et al. J. Chem. Soc. (1958) 276; ibid. (1958) 1403, metal complexes of such ligands have hardly been investigated to date and only a few have been manufactured. The main reason for this is that no separating and purification processes for water-soluble complexes of this type have been available so far. For this reason, in spite of the efficiency of the hydroformylation process described in DE-PS 26 27 354, other commercial processes with sulfonated phosphanes as a catalyst component have not been able to establish themeselves, nor has the chemical industry been able to develop complex compounds containing water-soluble phosphanes as ligands. A literature search provides impressive substantiation of this situation. Wilkinson et al (Nouv. J. Chim. 2 (1978) 137) reports on water-soluble complexes of the ligand (C.sub.6 H.sub.5).sub.2 P(m--C.sub.6 H.sub.4 SO.sub.3 Na) which, however, sometimes occur as hydrates and whose structures have not been conclusively established.
The experiments conducted by Patin et al (Tetrahedron Lett. 28 (1987) 2507) on the synthesis of the triphenylphosphane trisulfonate derivative ClRh[P--(C.sub.6 H.sub.4 --m--SO.sub.3 Na).sub.3 ], which has an analogous formula to the known complex ClRh[P(C.sub.6 H.sub.5).sub.3 ].sub.3, resulted in a mixture of several compounds. This meant that it was impossible to define the catalytic activity of the individual components in the hydrogenation of unsaturated carboxylic acids. The fact that the method inherently involves the formation of phosphane oxide O.dbd.P(C.sub.6 H.sub.4 --m--SO.sub.3 Na).sub.3 seems to be particularly prejudicial to catalysis (Patin, l.c. and J. Mol. Catal. 44 (1988) 191).
A recent dissertation on the synthesis of organonickel complexes of trisulfonated triphenylphosphane also failed for lack of a method to separate the reaction products (A. Sivade, Ph. D. thesis dated 11/13/87, Universite Paul Sabatier, Toulouse, France).
Therefore, the problem was to develop a process for the separation of the components of an aqueous solution containing metal complex compounds, free ligands, and other compounds in as simple a manner as possible. This process is to permit both the purification e.g. of metal complex compounds and organoelemental ligands and the regeneration of catalyst systems with metal complex compounds as active components.